Isovalent Impurities
Besides silicon, group IVa of the periodic system contains also the isovalent elements carbon, germanium, tin, and lead. Carbon, the lightest of them, is an omnipresent impurity introduced often unintentionally. On the other hand, as discussed in Section
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Isovalent Impurities Besides silicon, group IVa of the periodic system contains also the isovalent elements carbon, germanium, tin, and lead. Carbon, the lightest of them, is an omnipresent impurity introduced often unintentionally. On the other hand, as discussed in Section 4.1, it is used for a variety of applications from stress compensation to proximity gettering. The properties of germanium in silicon are summarized in Section 4.2. Unfortunately, the space would not have sufficed for a presentation of silicon-germanium materials in general. Tin, the last isovalent impurity considered here, has already limited technical applications. However, it was studied to investigate basic diffusion processes in silicon. The main results will be presented in Section 4.3.
4.1
Carbon
Carbon is a very common impurity in silicon wafers, introduced either from the polysilicon used in crystal growth, or from contaminations during this process. Typical concentrations in silicon wafers used in VLSI technology are now below 1016 em>' , but can be one or two orders of magnitude higher in solar-grade silicon [4.1] and up to 2.10 18 cm' in intentionally doped crystals [4.2]. Epitaxial deposition or solid-phase epitaxial regrowth of carbon-doped layers allows even the incorporation of carbon in a concentration on the order of 1021 cm" into Si and SiGe [4.3-4.7]. In addition, carbon may be introduced intentionally or unintentionally during process steps. Typical examples for the both would be ion implantation of carbon and dry etching, respectively. Carbon, as outlined below, resides predominantly on substitutional sites in the silicon lattice. Of special importance is the reaction with silicon self-interstitials which displaces substitutional carbon atoms to interstitial sites where they are highly mobile. Carbon interstitial s subsequently form a variety of complexes with intrinsic point defects, substitutional carbon, and dopants. Substitutional carbon atoms otherwise remain relatively inert and are only known to form complexes with oxygen atoms. The formation of carbon interstitials and their reactions with other point defect means effectively the removal of one silicon self-interstitial per carbon atom displaced from its substitutional site. In consequence, substitutional carbon is able to provide the free volume needed for the nucleation of oxygen precipitates [4.8-4.12] and for an enhanced formation of new donors [4.13-4.16]. Probably due to reactions with oxygen , but most likely also because of its influence on the intrinsic point defects, carbon was reported to cause a reduced formation of thermal donors [4.15-4.19]. More important now, the oversaturation of self-interstitials during post-implantation anneals can be reduced significantly by the presence of implanted carbon [4.20-4.24] or carbon incorporated during the epitaxial growth of silicon layers [4.25-4.29]. 281 P. Pichler, Intrinsic Point Defects, Impurities, and Their Diffusion in Silicon © Springer-Verlag Wien 2004
CHAPTER 4. ISOVALENT IMPURITIES
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